The present invention relates to a small signal input circuit which integrates a small input signal for a certain time length and transmits the integrated signal as input data and, more particularly, to an integral-type input circuit suitable for use in an X-ray CT system for receiving X-ray image signals.
It is a common practice in X-ray CT systems to receive a small current signal from each X-ray sensor by integrating it for a certain period and to provide an amplified voltage signal. The sensor current generally includes an offset current, such as a leakage current, which causes error in the integrated signal. Therefore, some compensating means for the offset current is needed in order to provide an accurate integration output for the input current signal.
U.S. Pat. No. 4,163,947 discloses an integration circuit provided with means for compensating the input offset current. This circuit has a compensation amplifier which supplies a current corresponding to a voltage held by an input capacitor to the input of the integration amplifier. During the compensation period between periods of integration, the compensation amplifier is coupled to the feedback loop of the amplifier so that the capacitor voltage follows the input offset current, thereby supplying a compensation voltage held in the capacitor for canceling the offset current to the integration amplifier during the integration period. Although this compensating means functions to compensate an input offset current which varies slowly enough to be regarded as a direct current as compared with the iterative period of integration, if the offset current includes a sinusoidal component, the capacitor voltage will follow this component, resulting in an erroneous voltage held in the capacitor, and accurate compensation cannot be expected. Particularly, the input circuit of the X-ray CT system is susceptible to the interference of a relatively low frequency noise current created by the mechanical vibration due to the rotation of the sensor in addition to the slow-varying leakage current, and therefore, the abovementioned conventional integration circuit does not meet the requirement as an input circuit of the X-ray CT system.